Vigenère cipher

# title: Vigenère cipher
# source: https://rosettacode.org/wiki/Vigen%C3%A8re_cipher
# category: Rosetta Code
# difficulty: Medium
# summary: Encode and decode a short uppercase message with a fixed Vigenère key.
# tags: popular, strings, ciphers, encoding
# vibe: 0.2

def alphabet
  "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
end

def shift_char(char, key_char, direction)
  char_index = alphabet.index(char)
  key_index = alphabet.index(key_char)

  if direction == "encode"
    alphabet.slice((char_index + key_index) % 26)
  else
    alphabet.slice((char_index - key_index + 26) % 26)
  end
end

def apply_vigenere(text, key, direction)
  output = ""
  key_index = 0
  index = 0

  while index < text.length
    char = text.slice(index)
    if char == " "
      output = output + char
    else
      output = output + shift_char(char, key.slice(key_index % key.length), direction)
      key_index = key_index + 1
    end
    index = index + 1
  end

  output
end

def run
  plain = "VIBESCRIPT ROCKS"
  key = "CODE"
  encoded = apply_vigenere(plain, key, "encode")
  {
    encoded: encoded,
    decoded: apply_vigenere(encoded, key, "decode")
  }
end